[WEB-4683] Overhaul of UI for app header, navigation, ToC, page header and footer

[jamiehenson]

This PR is culmination of a series of already-reviewed PRs upgrading the UI/UX of various key structural aspects of the docs site, namely the:

• Header ([WEB-4684] Core navigation component redesign (LeftSidebar, Header, RightSidebar) #2952)
• LeftSidebar ([WEB-4684] Core navigation component redesign (LeftSidebar, Header, RightSidebar) #2952)
• RightSidebar ([WEB-4684] Core navigation component redesign (LeftSidebar, Header, RightSidebar) #2952)
• PageHeader ([WEB-4686] Add PageHeader component and migrate LanguageSelector to Radix UI #2956)
• Footer ([WEB-4100] New Footer component for refreshed nav #2374)
• Internal table and admonition components ([WEB-4801] Add Admonition and Table MDX components #2989)

This represents a huge change for the look and feel of the docs site, moving us to a superior navigational experience that's more inkeeping with the quality of our competitors. That's corpospeak for Jamie W and Mark done good.

That said, this work also allows us to streamline things technically. I've taken the opportunity to lean more heavily on Radix UI for the foundational of these components, while also decoupling from Ably UI where possible and also improving the capabilities of the IA that powers the nav.

Give it a test run here: https://ably-docs-web-4684-docs-cdifa4.herokuapp.com/

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch web-4684-docs-redesign

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[m-hulbert]

This looks awesome 😎

[kennethkalmer]

YAS